1. Field of the Invention
The present invention relates generally to a method of recovering a dropped call in a mobile communication system, and in particular, to a device for rapidly recovering a dropped call using a physical channel additionally provided for multimedia services in the future mobile communication system.
2. Description of the Related Art
Conventional CDMA (Code Division Multiple Access) mobile communication technology includes the IS95A/B standards and the JSD-008 standards. According to the standards, if a mobile station declares receipt of 12 consecutive bad frames during decoding, it disables its transmitter. While in this state, if 2 consecutive good frames are decoded, the mobile station enables the transmitter and performs a normal communication. On the other hand, if the mobile station fails to receive 2 consecutive good frames for five second after it disables the transmitter, it releases the communication with a base station.
To recover the dropped call, the user of the mobile station should redial a telephone number. That is, the mobile finds a base station whose signal is received with good quality using a searcher and then establishes a new call through receipt of a synch channel, receipt of a paging channel, access probing, and establishing a traffic channel in sequence for communication with the new base station.
However, a total of ten seconds is allotted to attempt a call recovery after five seconds. This is a long time for a subscriber to wait. Therefore, if consecutive frames are damaged for a predetermined short time (12 or more frames), the mobile station searches its adjacent base stations using the searcher and establishes the new call with the searched base station. FIG. 1 illustrates a conventional procedure of searching adjacent base stations using a searcher in a mobile station when consecutive frames received for a predetermined time are defective.
Referring to FIG. 1, the mobile station determines which base station to communicate with (hereinafter, referred to as a serving base station) in a system determination substate in step 101 and enters a pilot channel acquisition substate in step 102. In the pilot acquisition substate, the mobile station acquires a pilot channel and the short PN code timing of the system. In step 103, the mobile station enters a sync channel acquisition substate. The mobile station receives and demodulates a sync channel message based on the acquired short PN code timing and analyzes the sync channel message to obtain initial system information such as information about its adjacent base stations and timing information. The mobile station then transits to a timing change substate and synchronizes its timing to the system timing in step 104. When an initialization state is over, the mobile station enters an idle state in step 105. While in this state, if a call is initiated the mobile station enters a system access state and attempts to access the serving base station in step 106.
Upon entering a traffic channel initialization substate in step 107, the mobile station requests the serving base station to assign a traffic channel. The mobile station transits to a waiting for order substate in step 108 and then to a waiting for mobile station answer substate in step 109. When the traffic channel is assigned for the call, the mobile station enters a conversation substate to perform the call in step 110. If the call is completed normally, the mobile station releases the traffic channel in a release substate in step 1111 and returns to step 103 for entering the sync channel acquisition substate.
In the conversation substate, the mobile station activates a call drop timer. If a predetermined number of consecutive good frames are not received before the call drop timer expires, the mobile station transits to a waiting for search task report substate and searches for an adjacent base station with the highest signal strength measurement in step 112. If the mobile station acquires the new base station, it transits to the idle state for the serving base station in step 105 and to the sync channel acquisition substate for the new base station in step 103. The mobile station is assigned to a traffic channel by the acquired base station to recover the dropped call. If the call is completed normally, the mobile station transits to the sync channel acquisition substate in step 103. On the contrary, if the dropped call is not recovered but released, the mobile station should return to step 101 to reacquire a base station in the system determination substate.
FIG. 2 illustrates the conventional dropped call recovery procedure in the mobile station. Referring to FIG. 2, the mobile station receives a sync channel and synchronizes its timing to system time in step 201. In step 203, when a call is originated, the mobile station accesses the serving base station and requests the base station to assign a traffic channel. Then, the mobile station is assigned to the traffic channel by the base station in step 205 and performs the call on the assigned traffic channel in step 207. If the call is dropped due to receipt of 12 consecutive bad frames, the mobile station disables its transmitter in step 209.
In step 211, the mobile station performs traffic channel supervision to determine whether 2 consecutive good frames are received for a predetermined time, for example, 5 seconds. If two consecutive good frames are received, the mobile station enables the transmitter and performs the call. Otherwise, the mobile station searches for a base station other than the serving base station, of which the signal is received with an acceptable level of signal strength using a searcher. If the mobile station succeeds in finding the new base station, it gives up communicating with the serving base station, receives in step 213 a sync channel from the new base station, synchronizes its timing to the new base station time, and originates a call in step 215. Then, the mobile station is assigned to a traffic channel by the new base station and recovers the dropped call in step 217.
The above conventional dropped call recovery method, however, has the problem that the mobile station must initiate a call attempt starting from the sync channel acquisition substate for the new base station. In this case, the mobile station releases all physical establishment on the traffic channel in use, for example, releases assignment of a finger receiver and reassigns a finger receiver to the new base station. If the mobile station receives a signal of good quality from the old base station, it fails to perceive the successful receipt of the signal. This implies that despite probable resumption of the call with existing resources, repetition of the traffic channel assigning procedure delays the recovery of the dropped call.